1. Field of the Invention
The present invention relates to semiconductor optical devices, and more particularly, to a semiconductor optical device having a current-confined structure.
2. Description of the Related Art
Typically, semiconductor optical devices are applicable in various fields owing to their long lifetime and high optical conversion efficiency. In particular, the semiconductor optical devices have attracted considerable attention as active devices such as light sources, for communications with a wavelength range of 1.2 to 1.7 μm, and detectors. In such a semiconductor optical device, a gain region is formed between semiconductor layers of opposite types, i.e., a p-type semiconductor layer and an n-type semiconductor layer, which are typically grown over InP or GaAs substrates, and a current-confined structure is formed in at least one of the semiconductor layers. Then, electrodes are formed on the upper and lower semiconductor layers so as to operate the optical device. The current-confined structure performs a very important function for the operation of the optical device.
The current-confined structure of the semiconductor optical device can be formed by various conventional methods. For example, a method of laterally oxidizing an Al(Ga)As semiconductor layer using wet oxidization, a method of laterally oxidizing an InAlAs layer or an AlAsSb layer using wet oxidization, a method of forming an undercut current-confined region by etching a semiconductor layer, a method of performing an ion implantation process and an annealing process, and a method of etching an adjacent portion of a tunnel junction and performing a re-growth process have been employed.
However, the foregoing methods of forming a current-confined structure of a semiconductor optical device have the following problems.
Firstly, a semiconductor optical device having a current-confined structure that is formed by laterally oxidizing an AlAs layer using wet oxidization has excellent characteristics. However, in this method, a gain medium grown over a GaAs substrate cannot reliably provide a wavelength range of longer than 1 μm. Therefore, a new method of forming a current-confined structure applicable in a long-wavelength range of approximately 1.55 μm is required.
Secondly, a current-confined structure, obtained by laterally oxidizing an InAlAs layer or an AlAsSb layer using wet oxidization, is commercially adverse because the wet oxidization should be performed at a high temperature of 500° C. for quite a long time and a crystal growth including Sb is required. Thirdly, the method of forming an undercut current-confined region by selectively etching a semiconductor layer makes the semiconductor optical device mechanically unreliable.
Fourthly, when a current-confined structure is formed using an ion implantation process and an annealing process, it is difficult to cure crystalline structures formed in an unwanted region. Finally, the method of etching an adjacent portion of a tunnel junction and performing a re-growth process is very complicated and tends to get good results only from optical devices grown using commercially adverse molecular beam epitaxy.